ALIMENTARY CANAL. 



The alimentary canal of Echini consists of an oesophagus, a so-called 

 stomach proper, and a terminal intestine leading to the anal opening. In 

 the Desmosticha and Clypeastroids the oesophagus is further specialized ; at 

 the entrance of the actinostome it is divided into a narrow canal, situated 

 entirely within the pyramids of the jaws ; this narrow oesophagus is more or 

 less pentagonal near the actinal opening, strongly ribbed by five longitudinal 

 bands with folds intermediate between them,* extending from the five basal 

 lips attached to the teeth proper, and the junction of the narrow oesophagus 

 with the oesophagus proper. In the Desmosticha the alimentary canal (if we 

 place the test so that it winds, after leaving the upper part of the pyramid, 

 from left to right) follows the outline of the test, making a complete cir- 

 cuit ; it then turns upward towards the abactinal pole, and curves back again 

 in the opposite direction, forming a second circle round the test from right to 

 left. These circles are not simple circles, but successive loops pointing towards 

 the apical system in all the interambulacral spaces, and towards the actino- 

 stome in the ambulacra] spaces. These loops thus form two concentric 

 series, extending round the test (PL XXVII f. 1. s, 4, 5 ; PI XXVIII. f. 5), 

 and terminating in the narrow terminal anal intestine, which runs obliquely 

 towards the apical system to the anal system [PL XXVIII. f. 3, 6, a). 



The alimentary canal is kept in position by minute, flat-pointed appendages 

 on the edges of the canal, firmly attached to the inner surface of the test. 

 These points occur at regular intervals, and are particularly well shown on 

 PI. XXVII f. 5 ; PI XXVIII f. s, 6 ; towards the abactinal part of the 

 alimentary canal they coalesce, and gradually pass into a rounded thread, 

 which follows the course of the alimentary canal (PI. XXVII. f. ,j, 4). The 

 terminal intestine flares, somewhat trumpet-shaped, immediately below its 

 point of attachment to the anal system, to which it is attached by prominent 

 radiating bands, similar to the mesenteries of the actinal part of the alimentary 

 canal (PI. XXVIII f. s, 6). The upper part of the oesophagus is remarkable 

 for the papillae, which cover the whole exterior surface (PI. XXVII. f. 1, 5). 



* Valentin, Anatomie du Genre Echinus, PL VII. f. 124. 



